A
sonority hierarchy or
sonority scale is a ranking of
speech sounds (or
phones) by amplitude. For example, if one says the
vowel [a], he or she will produce a much louder sound than if one says the
stop [t]. Sonority hierarchies are especially important when analyzing
syllable structure; rules about what
segments may appear in
onsets or
codas together, such as
SSP, are formulated in terms of the difference of their sonority values. Some languages also have
assimilation rules based on sonority hierarchy, for example, the Finnish
potential mood, in which a less sonorous segment changes to copy a more sonorous adjacent segment (e.g.
-tne- → -nne-).